i) Field of the Invention
The present invention relates to a light-emitting device, a method for producing the same, and a fluorescent device. More specifically, the present invention relates to a light-emitting device using an organic fluorescent material and the like, a method for the producing the same, and a fluorescent pattern-formed device.
ii) Related Art
In the illumination market, from the aspect of diversification of usage or use environment, requirement for design property, and environmental consciousness, LED is increasingly expected as a new light source. The emission efficiency of LED using an inorganic fluorescent material in the fluorescent layer is being drastically enhanced at present and is estimated to eventually surpass the emission efficiency of a fluorescent lamp. However, in the case of using LED for a lighting system, depending on the usage, excellent characteristics are required not only in the emission efficiency but also in the color rendering.
As for the current white LED system, a combination of a blue light-emitting part (LED portion) and a yellow light-emitting fluorescent material is predominating. However, in such a combination, the light emission of the red component is weak and the red color rendering can be hardly increased.
In order to solve this problem, a method of adding a red fluorescent material may be considered, but under present circumstances, the characteristics of the inorganic red fluorescent material are not necessarily satisfied and a white light-emitting LED comprising only an inorganic fluorescent material having added thereto an inorganic red fluorescent material cannot obtain sufficiently high light emission intensity.
On the other hand, it has been proposed to use an organic fluorescent material as the red fluorescent material for LED (see, JP-A-2005-15564). From this organic fluorescent material, red light with high light emission intensity can be obtained. In the case of intending to produce a white LED device by using such an organic fluorescent material as the red fluorescent material, an inorganic fluorescent material needs to be used in combination to realize white color, because as for the light emission other than red color, satisfactory characteristics are not obtained at present by an organic fluorescent material.
For example, there has been proposed a light-emitting device obtained by disposing a stacked body in which a layer comprising an inorganic fluorescent material and a layer comprising a fluorescent material containing a rare earth ion complex (inorganic fluorescent material) are separated from each other, such that the layer comprising an inorganic fluorescent material is positioned closer to the semiconductor light-emitting element (see, JP-A-2004-356358). According to this light-emitting device, it is indicated that the problem of accelerated deterioration of the rare earth complex resulting from local heating of the rare earth ion complex-containing fluorescent layer due to heat generated by light emission of the semiconductor light-emitting element can be prevented. However, in a light-emitting device comprising such a stack-type fluorescent layer, there may arise separation due to poor adhesion, difference in the thermal expansion coefficient, or the like among a plurality of fluorescent layers. Also, due to difference in the refractive index, fluctuation of the interface state, or the like among a plurality of fluorescent layers, the light extraction efficiency may decrease.
As a matter of course, in the case of using a fluorescent layer comprising only an organic fluorescent material (e.g., rare earth ion complex) as the fluorescent material, there is a problem that the heat generated by light emission of the semiconductor light-emitting element causes local heating of the fluorescent layer and this accelerates deterioration of the organic fluorescent material.
As described above, in a light-emitting device where a semiconductor light-emitting element is provided by stacking an inorganic fluorescent layer and an organic fluorescent layer, separation may arise due to poor adhesion, difference in the thermal expansion coefficient, or the like among a plurality of fluorescent layers, or the light extraction efficient may decrease due to difference in the refractive index, fluctuation of the interface state, or the like among a plurality of fluorescent layers, and the reliability is low.
Also, in the case of using a fluorescent layer comprising only an organic fluorescent material as the fluorescent material, since the fluorescent layer is locally heated due to heat generated by light emission of the semiconductor light-emitting element, deterioration of the organic fluorescent material is accelerated.